1. Field of the Invention
The present invention relates to a semiconductor device in which group II-VI compound semiconductors are used and, more particularly, it relates to a semiconductor device which includes a group II-VI compound semiconductor layer of the p-conductive type.
2. Description of the Related Art
Some group II-VI compound semiconductors including ZnS, ZnSe and the mixed crystal of ZnS and ZnSe have a relatively wider forbidden band, as compared with those of other well-known semiconductors. Therefore, attention has recently been paid to these group II-VI compound semiconductors as materials by which various optical semiconductor devices including light-emitting diodes, semiconductor lasers and photodiodes which emit light ranging from visible radiation to ultraviolet may be made.
In order to make these optical semiconductor devices, it is necessary to form a p-type conductive layer high in carrier density but low in resistance. According to the most popular knowledge about controlling the type of conductivity of the semiconductor, group I or V atoms are suitable for use as p-type impurities for the group II-VI compound semiconductors. Although various crystal growths have been tried and various impurities have been added, a p-type conductive layer high in carrier density and low in resistance has not yet been made in the case of the group II-VI compound semiconductors whose forbidden band is wide. The above-mentioned optical semiconductor devices cannot be made yet using the group II-VI compound semiconductors of wide forbidden band.
The tendency that it becomes more difficult to form the p-type conductive layer of high carrier density as the forbidden band becomes wider is not inherent solely in the group II-VI compound semiconductors but commonly found in group III-VI compound semiconductors. It has been supposed that this phenomenon is due to the self-compensating effect in which the energy stabilizing effect created by electrons accepted and given between impurities added and defects inherent in the crystal formed becomes larger than energy loss needed to create the inherent defects as the forbidden band becomes wider. This effect is essential and it has therefore been regarded as being impossible that the p-type conductive layer of high carrier density can be made by the group II-VI compound semiconductors of wide forbidden band because of the self-compensating effect.
However, it has been quite recently confirmed that the p-type conductive layer can be made by adding group I or V atoms even in the case of ZnSe and that the self-compensating effect can be avoided. A notable example telling us that the p-type conductive layer of high carrier density could be made by adding Li atom of the group I to ZnSe can be found in Yasuda, et al, "Metalorganic vapor phase epitaxy of low-resistivity p-type ZnSe", Applied Physics Letter, Vol. 52, page 57 (1988). According to this essay, an Li-added ZnSe layer was epitaxial-grown on the crystal of a GaAs substrate at a temperature of 450.degree. C. according to the MOCVD method and a p-type layer having a carrier density of 3.times.10.sup.16 -9.times.10.sup.17 cm.sup.-3 was made. This carrier density reported was obtained by the measurement of Hall effect and in the case of measuring the Hall effect about a crystal of heterojunction having the ZnSe layer piled on the GaAs, the carrier density is measured higher than its true value because of the inter-diffusion of atoms at the border face of the heterojunction and the effect of secondary electron gases. It was therefore a little doubtful whether or not the high carrier density of 9.times.10.sup.17 cm.sup.-3 was true. However, DePuydt, et al, "Electrical characterization of p-type ZnSe", Applied Physics Letter, Vol. 55, page 1103 (1989) reports that a p-type conductive layer having a carrier density of 1.times. 10.sup.17 cm.sup.-3 or lower can be made when the same Li-added ZnSe layer is grown according to the MBE (or Molecular Beam Epitaxial) method, and it has been confirmed that the p-type conductive layer can be made to have a relatively high carrier density when the ZnSe crystal is formed by adding Li to it.
However, it is well-known that the Li atom is an impurity quite easily diffused, and Cheng, et al, "Growth of p- and n-Type ZnSe By Molecular Beam Epitaxy," Journal of Crystal Growth, Vol. 95, page 512 (1989) concretely reports that the Li atoms diffuse even in the course of crystal growth and that it is difficult to control them so as to be distributed in the crystal as desired. The same test result was obtained by inventors of the present invention. Further, they have found that abnormal diffusion of Li atoms is caused and the Li atoms are remarkably piled on the border face of BaAs relative to ZnSe in the course of the crystal growth of the Li-added ZnSe on BaAs, and that the rate or activating rate of those electrically-active impurity atoms which are included in Li atoms taken into the crystal and which create p-type carriers (or holes) is by far lower than the predicted value or only about one tenth at the highest. It has become apparent, therefore, that Li atom is not proper as the impurity to form the p-type ZnSe layer.
A further example of growing the p-type ZnSe layer with N atoms added is reported by Ohki, et al, "Nitrogen Doped p-Type ZnSe Layer Grown by Metalorganic Vapor Phase Epitaxy", Japanese Journal of Applied Physics, Vol. 27, No. 5, page L909 (1988). The p-type carrier density reported in this case is 10.sup.14 cm.sup.-3 at the highest and this value is too low to practically use the p-type ZnSe layer. It is said that the thermal equilibrium solid solubility of N atoms relative to the group II-VI compound semiconductors such as ZnSe is extremely small and it is therefore natural that the p-type carrier density is so low when N atoms are added. The attempt of advancing the crystal growth of ZnSe while cracking ammonium, which is used as the N atom material, is disclosed in Taike, et al, "P-type conductivity control of ZnSe highly doped with nitrogen by metalorganic molecular beam epitaxy", Applied Physics Letter, Vol. 56, page 1989 (1990) is reasonable as a non-thermal equilibrium method of taking into the crystal those N atoms which are difficulty taken into the crystal from the viewpoint of thermal equilibrium. It is reported in this essay that a p-type carrier density of 5.6.times.10.sup.17 cm.sup.-3 was realized as the result of this attempt. Because the carrier density was measured in relation to the Hall effect and test results which were not matched with the high p-type carrier density are shown in this essay, it is quite doubtful whether or not the p-type conductive layer whose carrier density was about 10.sup.17 cm.sup.-3 was truly obtained by adding N atoms. In fact, no report that these test results or the high p-type carrier density of about 10.sup.17 cm.sup.-3 realized by the addition of N atoms has been confirmed has as yet been presented.
It is difficult to form the p-type group II-VI compound semiconductor layer, suitable for practical use, by atoms of the groups I and V which are supposedly optimum according to the conventional knowledge, but it is recently reported that the p-type ZnSe layer having a relatively high carrier density was obtained by adding O (or oxygen) atoms which belong to the same group as S and Se. According to common sense until now, however, O atoms added to the ZnSe layer displace Se atoms in the crystal, form only those electron states which are called isoeletronic traps, and cannot become shallow acceptor levels which create p-type carriers. It is however reported in Akimoto, et al, "Electroluminescence in an Oxygen-Doped ZnSe p-n Junction Grown By Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol. 28, No. 4, April, 1989, page L531 that an O-atom added ZnSe layer was grown on the GaAs substrate according to the MBE method, that the p-type layer having a carrier density of 1.2.times.10.sup.16 cm.sup.-3 was thus obtained and that the pn junction was formed to make a diode of the injection current light emitting type. The author of this essay insists that the reason why p-type carriers were obtained by adding 0 atoms resides in that the O atoms themselves formed shallow acceptor levels. The O atom is not an impurity so easily diffused as the Li atom of the group I is. Further, it is supposed that the 0 atom belongs to the same group as Se and that it is higher in solid solubility as compared with atoms of the group V. If the O atom can become the p-type impurity, it will become unexpectedly suitable for forming the p-type layer with group II-VI compound semiconductors. However, the blue light emitting diode reported in this essay has high operating voltage and current of 3.5V and 200mA, respectively, and it is operated only at a cryogenic temperature of 77K. It cannot help saying therefore that this diode is quite insufficient from the viewpoint of practical use.
As described above, the optical semiconductor devices such as the light-emitting diode, semiconductor laser and photodiode which made it necessary to form the pn junction with the p-type layer having a relatively high carrier density could not be made according to the conventional technique, using the group II-VI compound semiconductors of wide forbidden band. This is because it is relatively difficult to form the p-type layer, which has such a carrier density as needed, with these group II-VI compound semiconductors of wide forbidden band and also because the pn junction cannot be formed because of the quite fast diffusion of the impurity atoms even when the p-type layer is made to have a carrier density near to the value needed.